The present invention relates to a polyamide acid composition for preparing a polyimide, and more particularly relates to a polyamide acid composition which is improved in stability when a polyimide product is molded from a solution of a polyamide acid in an organic solvent (hereinafter referred to as "polyamide acid solution").
A polyimide has an excellent heat resistance and it is difficult to mold the polyimide itself due to its excellent heat resistance. Therefore, the polyimide product is, in general, prepared by molding a polyamide acid solution which is a precursor of the polyimide, and converting the polyamide acid to the polyimide.
As a process for converting the polyamide acid to the polyimide there are known (A) a process by evaporating a solvent in the polyamide solution by heating, and at the same time thermally converting the polyamide acid to the polyimide; and (B) a process by chemically converting the polyamide acid to the polyimide with chemical treating agent consisting of a dehydrating agent and a catalyst.
In the process (B), there are known two processes, that is, (B-1) a process by molding a polyamide acid solution, and then immersing into a converting bath prepared with a chemical treating agent; and (B-2) a process by molding a homogeneous admixture of a polyamide acid solution and a chemical treating agent.
These processes have, however, the following defects. As to the process (A), the heating should be carried out quite gradually in order to avoid generation of voids in the molding product, and water which is yielded during the conversion of the polyamide acid to the polyimide causes hydrolysis of the obtained polyimide to make characteristic properties of the polyimide reduce. As to the process (B-1), it takes a long time to convert the polyamide acid to the polyimide, and a degree of the conversion in the surface of the molding and that of the interior are not uniform. On the other hand, the process (B-2) does not have such defects as those in the processes (A) and (B-1). Therefore, the process (B-2) is superior in producibility and gives excellent characteristic properties of the molding. In the process (B-2), however, the conversion of the polyamide acid to the polyimide is drastically accelerated immediately after the chemical treating agents are added to the polyamide acid solution. As a result, the polyamide acid is gelled in a short time. Therefore, in case of employing the process (B-2), the molding operation, particularly continuous molding operation in preparation of film, is difficult and any uniform product cannot be obtained because the pot life of the polyamide acid composition is very short.
When the pot life is shorter than the time of period for molding operation, the pot life can be extended by altering an employed amounts of chemical treating agent. In that case, however, the producibility is lowered because a rate of conversion is lowered, and properties of the obtained polyimide are inferior to the polyimide prepared by the process (B-2).
In the above-mentioned convertional processes, in order to eliminate such defects, the polyamide acid composition is maintained at a low temperature. As a result, activities of chemical treating agent are reduced. By such manner, however, flowability of the polyamide acid solution decreases due to increase of the viscosity of the solution by cooling, and it is difficult to maintain the solution at a low temperature due to heat generation by admixing a chemical treating agent to the polyamide acid solution. As a result, enough pot life cannot be obtained. Therefore, the process of cooling also is not an effective treatment.
An object of the present invention is to provide a novel polyamide acid composition which can extend the pot life. According to the present invention, the pot life of the polyamide acid composition can be extended at normal temperature while maintaining the advantageous effects of the process (B-2).
According to the present invention, there can be provided a polyamide acid composition for preparing a polyimide comprising a solution of a polyamide acid in an organic solvent which is a precursor of the polyimide and an N-substituted heterocyclic compound of the formula (I): ##STR1## wherein A is a residue of a heterocyclic ring which constructs an N-heterocyclic ring; X is a member selected from the group consisting of ##STR2## Y is a member selected from the above group, hydrogen atom, an aliphatic group having 1 to 12 carbon atoms or an aromatic group; R.sup.1, R.sup.2, R.sup.3, R.sup.4, R.sup.5, R.sup.6 and R.sup.7 are the same or different and each is hydrogen atom, an aliphatic group having 1 to 12 carbon atoms or an aromatic group.
The composition is more stable at normal temperature in a form of solution than that in conventional process (B-2), and by using the composition the pot life can be properly extended and the characteristic properties of an obtained polyimide product are not reduced.